(Via Paolo Amoroso) The Logo Wiki is a great example of a code wiki. It needs some better version control of the code (in case of mistakes and typos) and I'm not sure how it deals with spam, etc. Still, it's fun and it makes it really easy to get a feel for Logo itself -- it helps that one of Logo's was (is?) teaching programming and that the interface is primarily a turtle carrying a pen!

Paolo Amoroso mentions that a Lisp variant would be even cooler and he's right in a way. I'm not sure that the examples would be as fun though.